We propose to continue to study the molecular events, regulation, and nature of molecules involved during the processes of platelet-platelet and platelet-endothelial cell interactions. Attempts will be made: to purify and further characterize the cAMP-dependent protein kinase of human platelets and to identify the platelet membrane protein receptor for this reaction; to characterize further the cultured human aorta endothelial cells and to establish further criteria for their identification as endothelial cells; and to explore further the effects of other aggregating agents on platelet-cultured endothelial cell interactions.